Field of the Invention
The present invention relates to an electric furnace steel for carburizing provided with carburizing quality equivalent to that of Cr—Mo steel without Mo addition and a manufacturing method for the electric furnace steel. The “electric furnace steel” means a steel manufactured according to a method of manufacturing steel using an electric furnace (hereinafter, referred to as “electric furnace method” as appropriate).
Description of the Related Art
Steel parts such as gears used for automobiles, industrial machines, and the like are required to have secured fatigue strength and abrasion resistance, in order to enable the steel parts to be used without replacement while receiving drive force from an engine and the like for a long period. Hence, alloy steel such as SCr and SCM having a carbon content of approximately 0.20% is used for conventional parts required to have fatigue strength and abrasion resistance, and a carburizing process that is a surface hardening process is performed thereon, whereby strength necessary for use is secured.
Normally, a Mo-added SCM material is used particularly for parts whose strength is strictly required. This is because Mo is an element that can improve hardenability while suppressing occurrence of an incomplete quenched structure of carburized surface, and Mo-added steel is thus effective to secure necessary hardenability regardless of part shapes and to manufacture parts having excellent strength.
However, in recent years, extreme fluctuations in price of raw materials necessary to manufacture special steel, such as scraps and ferroalloys that are raw materials of electric furnace steel, frequently occur. Fluctuations in price of ferromolybdenum necessary for Mo addition are no exception. Accordingly, in preparation for an extreme rise in price of ferromolybdenum, it is necessary to develop and prepare steel for carburizing that can secure properties equivalent to that of an SCM material without relying on Mo addition.
For example, Patent Document 1 describes a case-hardened steel that is developed with an object to secure bending fatigue strength and pitching strength equivalent to those of SNCM220H and SCM420H even in the case of avoiding containing Ni and Mo (expensive elements) as much as possible.
In Patent Document 1, in order to achieve this object, a deterioration of hardenability caused by reducing Ni and Mo contents is compensated for by making the component balance of Cr, Si, and Mn proper. Moreover, Patent Document 1 founds out that, in order to prevent bending fatigue strength and pitching strength from decreasing and minimize generation of coarse MnS that causes breaks during hot working and cold forging, it is important to make the balance of the Mn and S contents proper. Hence, coarse MnS generation is suppressed by performing such control that 30≦Mn/S≦150 is satisfied. Further, Patent Document 1 proposes that a problem that carburized abnormal layers such as an intergranular oxidized structure and an incomplete quenched structure are made deeper by avoiding adding Ni and Mo as much as possible is solved by setting the Cr, Si, and Mn contents to 0.7≦Cr/(Si+2Mn)≦1.1.
Moreover, Patent Document 2 describes a case-hardened steel that secures properties equivalent to that of Mo-added steel without adding Mo, even in the case of a rise in price of ferromolybdenum. In general, in order to suppress a deterioration of hardenability caused by a decrease in Mo, it is necessary to increase C, Mn, and Cr that are other hardenability improving elements than Mo, or add B that is another hardenability improving element. Unfortunately, if these elements are simply increased such that the hardenability becomes equivalent to that of the Mo-added steel, an incomplete quenched structure of carburized surface increases, and troostite in a carburized layer increases. Hence, it is difficult to secure properties equivalent to that of the Mo-added steel.
In view of the above, in Patent Document 2, the content of B, which is normally regarded as an impurity and is not particularly controlled, is strictly controlled, and the upper limit of B is set to be less than 0.0002%. This can suppress troostite generation in the carburized layer even without adding Mo. Further, Patent Document 2 discloses that the hardenability can be secured by satisfying Cr %−(Si %+Mn %+Cu %+Ni %+Mo %)≧0.30%.